The present invention relates to a cutting insert for chip forming machining tools, in particular milling cutters. The inserts are preferably produced by form-pressing and sintering of an insert forming powder. The insert comprises an upper chip surface, a lower planar bottom surface which is intended to be located in abutment with a cooperating bottom support surface of the machining tool, and at least one side surface extending between these surfaces. The side surface is adapted to be placed against at least one side abutment surface of the tool and is generally inclined at an acute angle in relation with the upper chip surface and at an obtuse angle in relation with the bottom surface. A cutting edge is formed along the transition line between the chip surface and the side surface, adjacent to which cutting edge a clearance surface is provided.
Such inserts are more and more produced by means of a direct pressing method at which a cemented carbide forming powder first is conferred the desired form in a pressing tool adapted for the purpose and subsequently is given final strength by sintering in an over at temperatures above 1000.degree. C. The pressing operation has been sophisticated over the years and is today so well defined that the process provides good possibilities of shaping the cutting edge, adjacent chip forming surfaces, possible reinforcing chamfers and clearance surfaces with large exactitude. Moreover, even shrinking that occurs during the sintering is included into the computation for the pressing tool dimensioning.
Today's cutting geometries tend toward more and more positive cutting geometries, i.e., larger and larger angles between the cutting insert chip surface and a normal plane of the machined surface. The reason for this development in the insert geometry is to provide the advantages achieved therewith, such as small cutting forces and a low energy consumption, a well-defined cutting edge for high dimension precision, as well as greater liberty when selecting the clearance angle, while maintaining positive cutting geometry. In practice, the limit for the positivity of the chip surface is set by the strength of the cemented carbide, since the cutting edge angle becomes sharper and thereby weaker the more positive the chip surface is.
Positive rake angles also entail that the axial inclination of the cutting insert in the milling cutter body shall be as positive as possible. However, a drawback with increasing inclination angles in the milling cutter body is that the clearance or relief angle diminishes with increasing cutting depths. As an example, indexable inserts with square basic form and with a side length of 1.5 cm and mounted with 7.degree. positive axial inclination in a milling body of 50 cm diameter have a clearance angle at the cutting insert corner of 10.degree. while the corresponding angle at maximal cutting depth is 7.degree.. If the same insert is leaned further in the same milling body, e.g., to 17 degrees positive axial angle, the clearance angle at maximal cutting depth diminishes to only 0.7.degree.. This decreased angle must be compared with the fact that a satisfactory clearance angle should be at least about 7.degree.. This inconvenience is further emphasized at small cutter body diameters.
It is well known to the person skilled in the art that sufficient clearance plays a decisive role for all cutting machining. Inadequate clearance with insufficient free play under the cutting edge results in an accelerated increase of flank wear on the insert and unacceptable vibrations. In addition, chipping, breakage or rupture of the cutting edge of the insert may occur. With the aim of trying to provide sufficient clearance at positive inclination of the inserts in the milling cutter body, a pressing of a helically twisted clearance surface under the cutting edge has been suggested. In this way, an essentially constant clearance toward the work piece is maintained despite the fact that the insert leans in the milling body.
However, a drawback of the twisted clearance is that a combination of a twisted clearance surface next to the cutting edge and a planar secondary clearance surface under the twisted clearance surface results in a transition or break line between these two clearance surfaces that is not straight and parallel with the cutting edge, but is curved. As a result of this curved break line, the width of the twisted clearance surface increases toward increasing cutting depth (see FIG. 1). This curved break line between the two clearance surfaces creates problems when determining where the abutment points or surfaces in the cutting insert pocket of the milling cutter body, for axial and radial positioning of the insert, shall be located. Moreover, for certain types of machining, for instance certain modes of face milling, the requirements on form and dimension precision have become more rigorous during recent years. In particular, positive cutting edges require a very high dimensional accuracy in order to guarantee a satisfactory function at small tool feeds. So far, these requirements on accuracy have been accomplished by so-called contour grinding, which means that each surface that adjoins the individual cutting edge is after-ground in a step after the sintering. However, a serious drawback of such a contour grinding is that it causes changes in the micro geometry of the cutting insert, i.e., in the surface structure of the cutting edge forming parts of the cutting inserts after a surface treatment, such as blasting, chamfering or deposition of a surface hardening layer, which surface treatment is normally effected as soon as possible after finished sintering. In this manner one may alter the width of occurring negative reinforcing chamfers, the distance from the cutting edge to the chip forming surfaces, as well as the clearance surface. For instance, a relief surface with an originally pressed, helically twisted shape will wholly or partly be ground away. In practice, these changes will likely cause the chip forming ability and chip forming function of the insert to deteriorate and its strength and tool life to be reduced.
Hence, a first object of the present invention is to avoid any form of grinding or other processing in the immediate proximity of the cutting edges.
Another object of the present invention is to enable an exact positioning of the axial and radial abutment points of a cutting insert in an insert pocket, in spite of a clearance surface which is not ground.
In a preferred embodiment, another object of the present invention is to enable an exact positioning of the axial and radial abutment points of the cutting insert in an insert pocket, even in the case of a helically twisted relief or clearance surface.
According to the present invention, these objects and others are realized. In accordance with the present invention any form of grinding or other processing is avoided in the immediate proximity of the cutting edges. In particular, by providing a recess in the side surface(s), the width of the clearance or relief surface becomes essentially constant, even when it is helically twisted. Furthermore, the clearance surface is not influenced in any way by the grinding of the underlying planar side surface, which therefore can be shaped in any desired manner in order to enable high dimensional accuracy at the axial and radial positioning of the cutting insert.
An additional advantage of the invention is that a large degree of liberty is made possible when selecting different clearances on the planar side surface without influencing the abutment height. Also, a further advantage is that the non-active cutting edges and their adjacent clearance surfaces do not press against the side support surfaces in the insert pocket. Therefore, no risk of damage to these surfaces in their non-active position exists.
Also according to the present invention, the clearance surface adjoining the cutting edge, also called the first clearance or relief surface, is preferably helically twisted. This twisting gives the further advantage of retaining the clearance angle substantially constant, despite the axial positive inclination of the cutting insert in the milling cutter body. However, the first relief could also be wholly planar and have a clearance angle of, for example, 5.degree. to 25.degree., and preferably 5.degree. to 15.degree..